Twin roll continuous casting of aluminum was developed into a commercial process in the early 1950s. Since then, the process has gained worldwide acceptance in the aluminum industry as an economical method for producing a wide variety of flat-rolled products. The twin roll casting process converts molten aluminum directly into thin cast strip suitable for cold rolling; thus effectively eliminating the ingot casting, sawing, scalping, reheating and hot rolling associated with the traditional die-cast ingot and hot mill method of production. This significantly reduces the capital investment required and also produces considerable savings in energy, consumables and manpower. These economic benefits give twin roll caster based plants a pricing advantage in the increasingly competitive world aluminum market.
In twin roll continuous casting, molten aluminum of a constant composition, temperature and level is degassed and filtered before being introduced into the "head box" of the casting machine. The head box is connected to an elongate planar pouring nozzle commonly known as the "tip" which distributes the metal between the twin rolls of the machine, the width of the nozzle determining the width of the cast strip. The exit of the planar nozzle is slightly upstream of the centerline of the twin rolls, thus the work rolls of the caster solidify and hot roll the aluminum in one process. This combination of solidification and hot rolling generates a substantial roll "separating force." In other words, this separating force tends to force the two work rolls apart, in opposite directions through the roll axes. For example, a typical 1700 mm wide, 1000 mm roll diameter foil stock caster may experience separating forces in excess of 2000 metric tons.
In order to withstand and offset such great separating forces, which typically are uniformly distributed over the width of the rolls, the rolls are constructed of extremely rigid metal alloys in a barrel-shaped configuration so that their middle portion, which experiences the greatest deflection, may deflect to a greater extent than their outer edge portions to result in a nominally flat roll surface. The rolls commonly have a relatively thick outer "shell" for contacting the molten aluminum which is fabricated from a hard alloy steel, such as a chrome nickel alloy steel. The extreme heat of the molten metal and large mass of such rolls, including their outer shells, requires internal cooling to withdraw heat at a sufficient rate, and to prolong their life.
Because of the variations in the casting environment, such as the composition of the alloys being cast, the thickness being cast, the speed at which the rolls are turned, the width of the particular sheet, the rate of cooling/solidification, and other factors, it is impossible to design a particular construction of the rolls or their shells to deflect in a way to produce the desired sheet flatness in all cases. Aluminum sheet is preferably roll cast slightly thicker in its center to allow the sheet to be self-centering during subsequent operations in a rolling mill. Specifically, it is generally desirable to have an approximately 0.5-1.5% greater thickness in the center of the sheet as compared to the edge thicknesses. This increased center thickness is referred to as the "crown" of the sheet. If this center portion is thinner than the edges, it is sometimes referred to as "negative crown." At present, there is a need for a fine control of the output thickness of the cast aluminum sheet when the above-mentioned factors are constantly being varied.
Controlling the temperature of work rolls is desirable for maintaining a preferred distance or "gap" between rolls during the roll casting operation. If the temperature of a work roll is permitted to increase, its circumference will increase due to its thermal expansion, reducing the thickness of the sheet being roll cast.
Besides controlling the overall temperature of work rolls, it is also desirable to control the temperature at various locations along the length of a roll. The center of a work roll tends to heat up and expand more than its ends, resulting in the formation of a thermally induced crown on the roll. This roll crown, which may be referred to as a "positive" crown, then results in a central indentation or "negative crown" on the sheet or strip being cast. As little as a ten-degree Fahrenheit differential between the center and the ends of a roll may cause a crown to develop.
A limited amount of positive roll crowning is desirable to offset the bending of the work rolls by the sheet being cast. However, excessive roll crowning will cause the sheet to be roll cast thinner in its center portion than at its edges, resulting in a negative sheet crown. This is undesirable when the sheet is to be cast flat, for example, when foil will be made from the sheet. Current internal work roll cooling systems are not able to provide greater cooling to the center of the roll than to its ends to control excessive crowning. In other words, the relationship between the amount of cooling water circulating in the center of the roll and the ends is typically fixed. Due to variable cooling conditions caused by the roll casting of different metals at different thicknesses, and other factors, excessive work roll crowning may still occur even with these internal cooling systems.
In U.S. Pat. No. 4,565,240 to Shibuya, et al., a continuous twin roll caster is shown which utilizes variable pressure underneath the midportion of the outer shell of the roll to elastically deform the outer shell to control the amount of crowning in the middle. Initially, the outer shell has a negative crown and the pressure to the underside of the center portion is at a maximum to produce a flat roll. As molten metal is introduced between the rolls, the outer shell attains a positive crown and thus the pressure is decreased accordingly to maintain the flat profile of the roll. This type of crown control for the rolls can only be accomplished with relatively thin outer shells or sleeves which are readily deformable upon application of hydraulic pressure. Furthermore, in Shibuya, et al., the amount of control of the crown is a relatively coarse adjustment with only a single pressure chamber in the middle.
In U.S. Pat. No. 4,721,154, issued to Christ, et al., a continuous casting machine for rapidly solidifying metal is shown. Molten metal is fed through a planar nozzle onto a traveling cooling surface for rapidly solidifying the metal. To produce a foil having variable local thicknesses, the spacing between the nozzle and the flexible cooling surface of the roll is varied. This is accomplished by applying variable pressures under the cooling surface of the shell to meter an outflowing mass of molten metal from the nozzle. Again, such control of the thickness of the finished foil can only be accomplished using a relatively flexible shell. In this case, the shell is made from a copper or copper alloy having a thickness in the range of a few millimeters.
Lastly, U.S. Pat. No. 3,757,847 to Sofinsky, et al., describes a roll cooling system including a central header the header controls the distribution of the cooling water into the roll. However, the header of this device acts to position the cooling water flow into and out of circumferential sections of radial passageways in the roll, and not laterally along the roll. Therefore, the header of Sofinsky, et al. acts to direct inlet cooling water to one section of the roll, and to direct outlet water through the other sections, while along the length of the roll in a given section, the flow is the same. Thus, this cooling system cannot achieve roll crown control. Furthermore, the cooling system of Sofinsky, et al., effectively, only cools one-third of the width of roll.
The patents to Christ, et al.; Shibuya, et al.; and Sofinsky, et al. are classed as roll mold casters which do not apply any pressure to the molten material in order to hot roll it. These roll mold casters are used when it is not required or desirable to achieve a finer microstructure in the cast strip, which can only be achieved by high pressure applied by the work rolls. The absence of feedback force on the rolls in roll mold casting devices allows the use of very thin, flexible shells or other cooling surfaces which can be elastically deformed using sub-surface fluid pressure. Such cooling systems control devices are unsuitable for very heavy, continuous roll casters having extremely thick and rigid shells which experience thousands of tons of separating force.
Additional disadvantages and distinctions over the present invention of previous continuous casting and molding machines could also be articulated. Thus, the foregoing should not be considered exhaustive in this regard.
Therefore, there exists a substantial need for an improved system to better control the crown of work rolls in roll casting machines, and thereby control the crown of sheet produced by such machines.